While #8's work in 750 ml bottles, they don't hold as tightly as #9's so they will tend to fail quicker (often after only a fraction of a year, in my experience). So for anything that you wish to age for some time, #9's are by far the best way to go.
This is good to know. Unfortunately I've already ordered the #8 corks. But I don't plan on keeping the bottles for very long, wink wink.
In the future I'll just stick with the #9 corks.
I'll keep you posted on how long the #8's last too.